The main functions of interface circuits are level connection and impedance matching, sometimes with simple signal processing or "maintenance", electrostatic protection, AC/DC isolation, and other functions. Common circuit interfaces include TTL, CMOS, ECL, RS232, RS485, optical isolation interface, coil coupling interface, etc. Shenzhen Youqin Electronics Co., Ltd. mainly engages in PCBA OEM and SMT chip processing businesses. In the process of purchasing various electronic materials for customers and testing and processing, TTL, RS232, and RS485 interface circuits are often used.
Below, a brief introduction is given to these three interface circuits:
TTL is the English abbreviation for Transistor Transistor Logic. From its name, it can be seen that the original intention of this interface level standard is to be used between digital systems based on transistor structures.
The standard power supply for the internal active components of a digital circuit working under the TTL interface standard should be 5V, and the output and input situations are as follows:
For the output terminal, the voltage requirement for state 1 is greater than or equal to 2.4V, and the voltage requirement for state 0 is less than or equal to 0.5V;
For the input end, the judgment requirement for state 1 is greater than or equal to 2.0V, and the judgment requirement for state 0 is less than or equal to 0.8V.
That is, it needs to be greater than a certain threshold to indicate level 1, and less than a certain threshold to indicate level 0.
Comparing the voltage requirements of the output and input terminals, it can be seen that the voltage output requirements of the output terminal are more stringent than the double threshold judgment criteria of the input terminal. This mainly takes into account the interference of noise and the transmission speed of electrical signals between output and input, thus making the double threshold judgment standard more reliable.
RS232
RS232 is a serial physical interface standard developed by the Electronic Industry Association (EIA) of the United States. RS is the abbreviation for Recommended Standard, meaning recommended standard in Chinese, and 232 is the identification number. The RS232 bus standard has a total of 25 signal lines, and we will only discuss its digital level interface judgment standard here.
The standard power supply for RS232 is ± 12V or ± 15V, the voltage requirement for state 1 is between -15V and -3V, and the voltage requirement for state 0 is between 3V and 15V.
RS485
RS485 is equivalent to an upgraded version of RS232. Similar to LVDS, RS485 also uses differential transmission to transmit information (although RS485 actually transmits two voltage signals), so its anti-interference performance is better than RS232. Here, we are also only concerned with the criteria for determining the digital level interface.
State 1 of RS485 requires a voltage difference of 2V to 6V between its two wires; State 0, the voltage difference between its two wires is required to be between -6V and -2V.
Shenzhen BQC often uses the above interfaces in OEM materials for customers, such as TTL and RS485 circuit interfaces for power bank customers, and serial UART for BeagleBone's development board based on TI's AM3352 chip. These interfaces are mainly used to connect computers for program burning, key burning, and testing various functions such as wired, wireless, Bluetooth, RTC, etc.
Can different standards be mixed and connected
The above introduces the interface level standards between various digital systems, and it is strongly recommended to choose a consistent standard for both parties of the digital system interface when using it. However, sometimes due to some configuration situations between the two parties, it may not be possible to find a unified level standard for communication. Therefore, is there no other method besides designing interface conversion circuit boards at this time? Not really, in fact, some different interface level standards are compatible.
Firstly, single ended and differential are not compatible because they are different in terms of physical connectivity. However, for the same type of interface, if the output of the A level standard meets the input of the B level standard, then the output of A can drive the input of B. If the reverse is also true, then the two level standards A and B can drive each other.
For example, the output of CMOS can drive the input of TTL, but the opposite is not true because the output of TTL state 1 is only greater than or equal to 2.4V, which cannot meet the requirement of CMOS decision state 1 to be greater than or equal to 3.5V; However, LVTTL3V3 and LVCMOS3V3 can drive each other because their outputs can meet each other's input judgment requirements.
